1. Technical Field
This disclosure relates to a thermosensitive recording material utilizing a color-developing reaction between an electron-donating coloring compound and an electron-accepting compound, or the like, more specifically, to a thermosensitive recording material for medical images.
2. Description of the Related Art
A thermosensitive recording medium is generally formed by providing on one surface of a support made of paper, synthetic paper or plastic film a thermosensitive color-developing layer composed mainly of a colorless or pale color-developing substance such as an electron-donating leuco dye, an organic acid color developer such as an electron-accepting phenolic compound, and a binder. It is possible to obtain a color-developing recording image by making the color-developing dye and the color developer react together utilizing thermal energy. Thermosensitive recording media like this are advantageous in that recording apparatuses for the thermosensitive recording media are compact and inexpensive, offer good maintainability and so forth; thus, the thermosensitive recording media are widely used for electronic calculators, facsimiles, automated ticketing machines, scientific measurers, CAD printers, plotters, printers for CRT medical measurement, and the like.
Amongst those uses, in the case of use where water resistance and tensile strength are required, in the case of use in image printers for CRT medical measurement where uniformity and high resolution of recorded images are required, and in the case of use in CAD plotters where dimension stability and thin line recording are required, thermosensitive papers are used in which synthetic paper having multilayer structure serves as a support.
In the field of medical treatment, states of the inside of bodies viewed using X-rays, MRI, CT scans, etc. have been made visible images on silver halide-based film, and those images have been looked at for visual diagnosis and referred to by means of backlight employed by view boxes. However, the wet process for the silver halide-based film has a problem with waste liquid disposal; further, along with the recent digitization of images, emergence of a dry process that replaces it has been demanded, and thermosensitive recording systems have already begun being used for reference in monitor diagnosis with digital images or for visual diagnosis with output of digital images as well as for CRT medical measurement.
Thermosensitive recording materials for medical uses are generally classified into the reflective type in which recording material has little or no light transmittance as a whole and a formed image is viewed by means of reflection of light, and the transmissive type in which recording material has light transmittance as a whole and the light transmittance is utilized; the present invention relates to the reflective type in which a formed image is viewed by means of reflection of light.
The characteristics required to allow for an image on thermosensitive paper to be used as a reflective-type medical image which is to be referred to or looked at for visual diagnosis include uniformity, high resolution, thin line recording, high glossiness, water resistance, curl reduction, measurement stability and tensile strength of a recorded image; therefore, a synthetic paper having a multilayer structure is used as a support.
However, synthetic papers having multilayer structure for thermosensitive papers used for electronic calculators, facsimiles, automated ticketing machines, scientific measurers, CAD printers and plotters are provided with concavities and convexities on their surfaces to improve printing suitability and writing quality that are necessary properties, and some of the convexities are protrusions of unsuitable height for support of thermosensitive recording media for reflective-type medical images. When a medical image is recorded onto a thermosensitive recording paper using any such support, there are problems caused in which white spots arise at halftone portions and solid image recording portions, so that uniformity decreases and so forth. Also, there is a problem in which the provision of concavities and convexities on a surface of a synthetic paper contributes to the lowering of glossiness, and high glossiness that a thermosensitive paper for reflective-type medical treatment is required to have so as to produce a photograph-like image cannot be yielded. Moreover, although the concavities and convexities are provided on the surface of the synthetic paper having a multilayer structure, the opposite surface thereof does not have the same structure, and thus there is greater curl when the synthetic paper is formed into a sheet, which is problematic when a medical image is observed.
In order to remove the white spots at the halftone portion and the solid image recording portion, Japanese Patent Application Laid-Open (JP-A) No. 3-190787 proposes and puts into practice a thermosensitive recording paper including: a support made of synthetic paper, and a thermosensitive color-developing layer on the support, wherein the support is a synthetic paper formed of a laminated film including a biaxially-stretched resinous film as a base layer and also including a uniaxially-stretched film of thermoplastic resin containing 10% by mass to 50% by mass of calcium carbonate powder as a paper-like layer on the front surface of this base layer, and the support satisfies the following properties (i) to (iii). (i) the opacity measured in accordance with JIS P-8138 is 45% or less; (ii) the Bekk smoothness of the paper-like layer onto which the thermosensitive color-developing layer is applied is 100 sec to 300 sec, and the center line average roughness (Ra) thereof is 1.5 μm or less; and (iii) the density of the support measured in accordance with JIS P-8118 is 1.1 g/cm3 or less.
Meanwhile, JP-A No. 07-81231 proposes a thermosensitive recording paper including: a support made of synthetic paper, and a thermosensitive color-developing layer on one surface of the support, wherein the support is a synthetic paper formed of a laminated film including a biaxially-stretched resinous film as a base layer, also including a paper-like layer formed of a uniaxially-stretched film of thermoplastic resin containing 1% by mass to 8% by mass of calcium carbonate powder on one surface of this base layer, and further including a back surface layer formed of a uniaxially-stretched film of thermoplastic resin containing 15% by mass to 55% by mass of inorganic fine powder on the other surface of the base layer, and the support satisfies the following properties (i) to (iv): (i) the opacity measured in accordance with JIS P-8138 is 45% or less; (ii) the Bekk smoothness of the paper-like layer onto which the thermosensitive color-developing layer is applied is 1,000 sec to 3,500 sec, and the center line average roughness (Ra) thereof is 0.5 μm or less; (iii) the Bekk smoothness of the back surface layer is 100 sec to 900 sec, and the center line average roughness (Ra) thereof is 0.6 μm to 1 μm; and (iv) the density of the support measured in accordance with JIS P-8118 is 0.91 g/cm3 to 1.1 g/cm3.
These supports can remove the problems to some extent in which protrusions of unsuitable height cause white spots to arise at halftone portions and solid image recording portions, so that uniformity decreases and so forth; however, they do not sufficiently meet requirements of reflective-type medical images, and high image glossiness required cannot be obtained either. Moreover, since the front and back of the base layer have different structures, there is greater curl.